Cyanides are useful materials industrially and have been employed in fields such as electro-plating and electro-winning of metals, gold and silver recovery from ores, treatment of sulfide ore slurries in flotation, tannery processes, etc. Due to environmental concerns, it is desirable to remove or destroy the cyanide present in the waste solutions resulting from such processes. Additionally, in view of the cost of cyanide, it is desirable to regenerate the cyanide for reuse in an efficient manner.
Techniques for cyanide disposal or regeneration (recovery) in waste solutions include: ion exchange, oxidation by chemical or electrochemical means, and acidification-volatilization-reneutralization (AVR). The terms recovery and regeneration are used interchangeably herein.
U.S. Pat. No. 4,267,159 by Crits issued May 12, 1981, discloses a process for regenerating cyanide in spent aqueous liquor by passing the liquor through a bed of suitable ion exchange resin to segregate the cyanide.
U.S. Pat. No. 4,708,804 by Coltrinari issued Nov. 24, 1987, discloses a process for recovering cyanide from waste streams which includes passing the waste stream through a weak base anion exchange resin in order to concentrate the cyanide. The concentrated cyanide stream is then subjected to an acidification/volatilization process in order to recover the cyanide from the concentrated waste stream.
U.S. Pat. No. 4,312,760 by Neville issued Jan. 26, 1982, discloses a method for removing cyanides from waste water by the addition of ferrous bisulfite which forms insoluble Prussian blue and other reaction products.
U.S. Pat. No. 4,537,686 by Borbely et al. issued Aug. 27, 1985, discloses a process for removing cyanide from aqueous streams which includes the step of oxidizing the cyanide. The aqueous stream is treated with sulfur dioxide or an alkali or alkaline earth metal sulfite or bisulfite in the presence of excess oxygen and a metal catalyst, preferably copper. This process is preferably carried out at a pH in the range of pH 5 to pH 12.
U.S. Pat. No. 3,617,567 by Mathre issued Nov. 2, 1971, discloses a method for destroying cyanide anions in aqueous solutions using hydrogen peroxide (H.sub.2 O.sub.2) and a soluble metal compound catalyst, such as soluble copper, to increase the reaction rate. The pH of the cyanide solution to be treated is adjusted with acid or base to between pH 8.3 and pH 11.
Treatments based on oxidation techniques have a number of disadvantages. A primary disadvantage is that no cyanide is regenerated for reuse. Additionally, reagent costs are high, and some reagents (e.g. H.sub.2 O.sub.2) react with tailing solids. Also, in both the Borbely et al. and Mathre processes discussed above, a catalyst, such as copper, must be added.
U.S. Pat. No. 3,592,586 by Scott issued Jul. 13, 1971, describes an AVR process for converting cyanide wastes into sodium cyanide in which the wastes are heated and the pH is adjusted to between about pH 2 and about pH 4 in order to produce hydrogen cyanide (HCN). The HCN is then reacted with sodium hydroxide in order to form sodium cyanide. Although the process disclosed in the Scott patent is described with reference to waste produced in the electro-plating industry, AVR processes have also been applied to spent cyanide leachate resulting from the processing of ores. Such spent cyanide leachate typically has a pH greater than about pH 10.5 prior to its acidification to form HCN.
AVR processes employed in the mineral processing field are described in the two volume set "Cyanide and the Environment" (a collection of papers from the proceedings of a conference held in Tucson, Ariz., Dec. 11-14, 1984), edited by Dirk Van Zyl. Also, see "Cyanidation and Concentration of Gold and Silver Ores," by Dorr and Bosqui, Second Edition, published by McGraw-Hill Book Company 1950, and "Cyanide in the Gold Mining Industry: A Technical Seminar," sponsored by Environment Canada and Canadian Mineral Processor, Jan. 20-22, 1981. Another description of an AVR process can be found in "Canmet AVR Process for Cyanide Recovery and Environmental Pollution Control Applied to Gold Cyanidation Barren Bleed from Campbell Red Lakes Mines Limited, Balmerton, Ontario," by Vern M. McNamara, March 1985. In the Canmet process, the barren bleed was acidified with H.sub.2 SO.sub.4 to a pH level typically between pH 2.4 and pH 2.5. SO.sub.2 and H.sub.2 SO.sub.3 were also suitable for use in the acidification.
AVR processes take advantage of the volatile nature of hydrogen cyanide at low pH. In an AVR process, the waste stream is first acidified to a low pH (e.g. pH 2 to pH 4) to dissociate cyanide from metal complexes and to convert it to HCN. The HCN is volatilized, usually by air sparging. The HCN evolved is then recovered in a lime solution, and the treated waste stream is then reneutralized. A commercialized AVR method known as the Mills-Crowe method is described in a paper by Scott and Ingles, "Removal of Cyanide from Gold Mill Effluents," Paper No. 21 of the Canadian Mineral Processors 13th Annual Meeting, in Ottawa, Ontario, Canada, Jan. 20-22, 1981.
A process using AVR to recover cyanide values from a liquid is described in Patent Cooperation Treaty application PCT/AU88/00119, International Publication No. WO88/08408, of Golconda Engineering and Mining Services PTY. LTD. The disclosed process involves treating a tailings liquor from a minerals extraction plant by adjusting the pH into the acid range to cause the formation of free hydrogen cyanide gas. The liquid is then passed through an array of aeration columns arranged in stages so that the liquid flowing from one aeration column in a first stage is divided into two or more streams which are introduced into separate aeration columns in successive stages. In a recent paper describing the process, it was stated that plant shutdown would occur if the pH went above pH 3.5. In a commonly assigned application, PCT/AU88/00303, International Publication No. WO89/081357, a process for clarifying liquors containing suspended solids is disclosed. The feed slurry is acidified to a pH of pH 3.0 or lower. Flocculants are added to cause the formation of flocs to enable the separation of the suspended solids from the liquor. The clarified liquor can then be used as a feedstock for the AVR process disclosed in the other commonly assigned application.
The AVR processes described in the Scott patent and the above-mentioned texts typically include the step of volatilizing HCN by contacting with air and then contacting the volatilized HCN with a basic material to convert HCN to a cyanide salt. The above-mentioned references also only disclose a treatment of barren bleed which typically results from Merrill-Crowe type cyanidation treatment of ore. This bleed does not contain solid tailings. Today many ores are treated by a carbon-in-leach or carbon-in-pulp cyanidation process. The tailings from such processes include the solid processed ore in the spent leachate. Typically the tailing slurries contain about 30% to 40% by weight solids and about 100 to 350 parts per million (ppm) cyanide. In the past, such tailings were typically impounded and the cyanide contained therein was allowed to degrade naturally. Due to environmental concerns about cyanide, such impoundment is not a desirable alternative in many situations. Therefore, it is often necessary to treat the material in some manner to decompose the cyanide. This is expensive due to the costs associated with the treatment, as well as the loss of cyanide values which results.
Therefore, it would be advantageous to extract and recycle cyanide from a cyanide-containing waste stream. Further, it would be advantageous to provide a process for treating cyanide-containing slurries which also contain ore tailings. It would be advantageous if the amount of cyanide present in the waste stream could be reduced. It would also be advantageous to regenerate the cyanide for reuse directly in the precious metals recovery circuit.
It has now been discovered that when the HCN is volatilized in the cyanide-containing waste stream, the HCN can be recycled to a cyanide recovery tower where it is contacted directly with a stream containing precious metals-containing ore, to recover precious metals therefrom. The use of such a process advantageously minimizes the input of bulk cyanide into the precious metals recovery system. The system can operate essentially as a closed system and does not require significant amounts of additional cyanide.
Further, the equipment and raw materials previously necessary for the reabsorption of cyanide into caustic solution is no longer required. This advantageously eliminates both equipment and raw material cost.